Lift Station 7 Rising Fast During a Night Storm
Station 7 serves 1,900 connections at the low end of a 12-inch gravity trunk. The station has two 450-gpm pumps in a lead/lag arrangement with automatic alternation; only one pump runs at a time. The wet well is 8 feet inside diameter in a 12-foot-deep barrel, with lead-on at 3.0 feet, lag-on at 7.0 feet, a high-level alarm at 10.5 feet, and the rim at 12.0 feet. On a dry day the lead pump runs 9 minutes out of every 20. During a 2.5-inch storm overnight, telemetry shows the lead pump running continuously and the level rising from 7.2 feet at 03:10 to 9.5 feet at 03:25. The alarm has not yet rung. The maintenance crew is 35 minutes away, the vacuum truck is at the yard, and there is no bypass connection at the nearest downstream manhole, 900 feet away. Upstream, crews have documented root intrusion at three joints, a 300-foot section running below 0.5 fps at dry-weather flow, and heavy grease in two manholes near a restaurant row.
1. Calculate the wet-weather inflow and the time remaining before the high-level alarm and before the wet well reaches the rim. What is your immediate action sequence, including notification obligations?
Read the worked answer
Wet well area = 0.785 x 8 squared = 50.3 square feet. Volume per foot = 50.3 x 7.48 = about 376 gallons.
Rise of 2.3 feet in 15 minutes = 865 gallons in 15 minutes = about 58 gpm of net accumulation while one 450-gpm pump runs continuously. Total inflow = 450 + 58 = about 508 gpm.
From 9.5 feet to the 10.5-foot alarm: 376 gallons at 58 gpm = about 6.5 minutes. From 9.5 feet to the 12.0-foot rim: 940 gallons = about 16 minutes. That is the entire response window.
Action sequence, in order: call the crew in immediately rather than waiting for the alarm; start the lag pump now - combined capacity of 900 gpm against 508 gpm of inflow will draw the well down - but first confirm the downstream force main and receiving manhole can take it, because if the constraint is downstream you are only moving the overflow. Dispatch a second person to the station. Stage the vacuum truck as a holding action; its realistic capacity is well above 58 gpm but it only buys time.
If a spill occurs, contain it (dike, sandbags, lime), keep it out of the storm drain, and notify the state water quality agency within the required time - in North Carolina that is prompt reporting and no later than 24 hours for a sanitary sewer overflow - plus the local health department and any downstream water users. Document start and stop times, estimated volume computed from pump run time and wet well dimensions, and the cause.
2. Using the dry-weather and wet-weather data, how do you separate inflow from infiltration, and what monitoring do you put in place?
Read the worked answer
Dry-weather average flow: the lead pump runs 9 minutes out of 20, so 0.45 x 450 gpm = about 202 gpm, or 0.29 MGD. Wet-weather flow is 508 gpm, about 2.5 times dry weather. A response ratio of 2.5 for a 2.5-inch storm is a substantial rain-derived component, not normal peaking.
The separation is a timing question, not a volume question. Direct inflow shows up within minutes to an hour of rainfall and decays quickly after the rain stops - it is roof leaders, area drains, broken or unsealed manhole covers, cross connections from storm sewers, and cracked frames and covers in the floodplain. Infiltration shows up hours to days later and stays elevated while groundwater is high - it is defective pipe, joints, and laterals below the water table. The telemetry on Station 7 already gives you the shape of the curve; what you need is the rain gauge next to it.
Install a temporary area-velocity flow meter in the trunk upstream of the station with a tipping-bucket rain gauge, and run it for at least eight to twelve weeks through several storms. That gives you the rainfall-to-flow relationship, the response lag, and the groundwater recession curve.
Then field work: manhole inspections in the floodplain, smoke testing on the residential laterals, dye testing of suspected storm cross connections, and CCTV of the three root-intruded joints and the flat 300-foot reach. Root control and grease control are cheap and immediate; the flat section is a candidate for CIPP or replacement because low velocity is what lets the grease and roots build up in the first place.
3. The utility manager proposes adding a third 450-gpm pump and raising the lead-on setpoint to solve the problem. Evaluate that proposal.
Read the worked answer
It addresses the symptom at the wet well and can make things worse downstream. The station is not short of pumping capacity at 508 gpm of inflow - one 450-gpm pump plus the lag gives 900 gpm, which is already more than enough to keep the well down. The reason the well rose is that only one pump was running. The capacity exists; the control logic and the response did not use it.
Adding capacity increases the peak discharge into the downstream gravity trunk. If that 12-inch trunk is already running full, a third pump just moves the overflow to a manhole 900 feet away, where it is harder to contain and harder to see. The correct analysis is a hydraulic model or at minimum a flow monitor confirming the downstream trunk and force main can pass the increased peak.
Raising the lead-on setpoint is worth doing - a larger operating band gives more emergency storage and fewer starts - but it is a margin improvement, not a capacity fix, and it must stay well below the high-level alarm.
The real fixes are, in order of cost effectiveness: an alarm autodialer and remote monitoring so the on-call operator knows at 03:10 rather than at 06:00; a written wet-weather staffing and bypass plan; source reduction of inflow through manhole cover sealing and lateral rehabilitation; and only then a capacity upgrade sized by actual flow data.
Put the proposal in writing with the flow numbers. The manager's instinct is understandable but the data do not support a third pump as the first spend.